Tumor Necrosis Factor‐α Activates Integrin Signaling in Vascular Smooth Muscle Cells via α5 Receptor

Abstract

Tumor necrosis factor‐α (TNFα) signaling requires superoxide (O2•−) to be produced extracellularly by NADPH oxidase 1 (Nox1) in vascular smooth muscle cells (VSMCs). Extracellular superoxide dismutase (ecSOD) converts O2•− to hydrogen peroxide (H2O2). It binds to the cell surface by association with either heparin‐sulfate or fibulin‐5, an extracellular matrix protein that interacts with integrin receptors. We previously found that ecSOD knockdown (siRNA) impaired TNFα signal transduction as reflected by MAPK and NF‐κB activation. Targeting of ecSOD also inhibited TNFα‐dependent activation of focal adhesion kinase (FAK) which reflects integrin receptor activation. Here we investigated the mechanism by which TNFα activates integrin signaling in VSMC. We analyzed NF‐κB activation in the presence of selective blocking antibodies (Ab) for integrin receptor subtypes previously shown to be expressed in VSMCs including α4, α5, αv, β1, and β3. TNFα‐induced NF‐κB activation was not affected by blockade of integrin receptors α4, αv, β1, and β3. However blocking of α5 reduced TNFα‐induced NF‐κB activation (control IgG + TNFα 7.36 ± 0.7, anti‐α5 Ab + TNFα 4.78 ± 0.3 fold change, p < 0.05, n = 5). We next sought to determine if α5 integrin is associated with upstream steps of integrin signaling in VSMCs. FAK and integrin‐linked kinase (ILK) are downstream effectors of integrin receptor activation. After 10 min, TNFα induced phosphorylation of both FAK and Protein Kinase B (Akt) which was used as a surrogate for ILK activation. Blockade of α5 receptors decreased TNFα‐induced phosphorylation of both FAK and Akt. Thus, short term exposure of VSMCs to TNFα transactivates α5 integrins and subsequently FAK and ILK‐Akt. We speculate that ecSOD localizes TNFα‐induced H2O2 production to the vicinity of α5 integrins via physical association with fibulin‐5 and this redox environment affects activation of the receptor.